Turn off deprecation errors in error.log file

These might be helpful: https://stackoverflow.com/questions/5628148/disable-e-deprecated-in-php-error-log and https://wordpress.stackexchange.com/questions/239501/suppress-deprecated-notices

This must be defined in a PHP setting. If your hosting does not offer you any options for this (ask your hoster’s support), you could try the following entry in wp-config.php:

ini_set('error_reporting', E_ALL & ~E_DEPRECATED);

However, this will only work if your hosting allows it. If you have any questions, you should also contact your hoster.

See also: https://stackoverflow.com/questions/5628148/disable-e-deprecated-in-php-error-log

You are aware why there are messages. Note that hiding the messages in the future can lead to you or someone else suddenly wondering why a plugin or theme no longer works properly because the deprecated function is no longer available and you wonder why there was never a message about it. My recommendation would be to contact the support of the component that caused this.

Many thanks for your replies.
I’m on shared hosting.
I also understand that current WP version is not fully compatible with 8.4 so I’ll wait a bit or simply re-test with version 8.3.